A prodrug activation strategy for selectively impairing tumor cells involves the expression of a gene encoding an exogenous enzyme in the tumor cells and administration of a substrate for that enzyme. The enzyme acts on the substrate to generate a substance toxic to the targeted tumor cells. This technique has advantages over the expression of directly toxic genes, such as ricin, diphtheria toxin, or pseudomonas exotoxin. These advantages include the capability to: 1) titrate cell impairment; 2) optimize therapeutic index by adjusting either levels of prodrug or of recombinant enzyme expression; and 3) interrupt toxicity by omitting administration of the prodrug. In addition, this technique uses prodrugs with different effects on different cell types, allowing treatment to be adjusted according to a specific disease state.
Enzymes useful in a prodrug activation approach have been described and include enzymes such as thymidine kinase, cytosine deaminase and purine nucleoside phosphorylase (PNP), as described in U.S. Pat. Nos. 5,338,678; 5,552,311; 6,017,896 and 6,207,150. However, the effectiveness of tumor treatment using prodrug activation techniques is limited in cases where side effects of substrate administration are present. For example, the prodrug ganciclovir, often used in combination with thymidine kinase, can cause unwanted immunosuppressive effects.
The search for a particular purine nucleoside phosphorylase with cleavage activity for the important chemotherapeutic F-araA has not previously been successful in part due to the large number of PNP candidates that need to be surveyed and the difficulties surrounding isolating and expressing each PNP. Many microorganisms generate PNPs capable of cleaving adenine-containing nucleosides to adenine. To illustrate, there are at least 17 microorganisms alone reported to express PNP including: Leishmania donovani; Trichomonas vaginalis; Trypanosoma cruzi; Schistosoma mansoni; Leishmania tropica; Crithidia fasciculata; Aspergillis and Penicillium; Erwinia carotovora; Helix pomatia; Ophiodon elongates (lingcod); E. coli, Salmonella typhimurium; Bacillus subtilis; Clostridium; mycoplasma; Trypanosoma gambiense; and Trypanosoma brucei. 
Thus, there exists a need for a prodrug activation method for treating tumors that improves efficacy and overcomes the problem of side effects.